Because conventional microfilm and microfiche recorded in photographic material deteriorate with time, it is desireable to record information for archival storage on a permanent medium. A surface relief pattern, e.g., a surface relief focused image hologram, provides such a permanent medium since it can be recorded on hard durable materials, e.g., glass, metal films and thermoplastics. Also, it is desireable to form a master capable of replicating surface relief holograms in a permanent material. Since conventional photoresists are relatively soft materials, a surface relief hologram formed in a photoresist must be transferred to a more durable material before it can be used as a permanent medium for archival storage or before it can be used as a master to replicate the surface relief pattern in a permanent material. The resolution required for the typical surface relief hologram is on the order of 1 micron. This high resolution necessitates that the transfer of a surface relief pattern from a photoresist to the material which is to form a master be accomplished with precision. The conventional technique for forming a master for a surface relief hologram is to deposit a metal film or hardenable material on the resist surface and separate the film from the photoresist surface. The film may be backed with a hard material for support. Great care must be exercised in separating the film from the photoresist to prevent damage to the surface relief pattern embossed on the film. U.S. Pat. No. 3,565,978 describes such a technique.
Until the present invention, it was possible to transfer a hologram recorded on a photosensitive surface layer covering a hard durable substrate material to the substrate material itself only by sputter-etching, as described in U.S. Pat. No. 3,773,258. "Conventional" etching techniques do not work well because the substrate can only be etched if it is revealed and if the substrate is only partially revealed after a single resist development, as in conventional etching techniques, the etch will not give a substantially linear transfer of the surface relief pattern. Sputter-etching is less efficient and less economical than chemical etching. Thus, it is desirable to develop a method for substantial linear transfer of a surface relief pattern recorded in a photoresist to an underlying hard durable substrate capable of forming a master hologram by chemical etching.